This invention relates generally to circuit breakers and, more particularly, to a power transformer with a small form factor.
Arc detection often is performed to protect house wiring and consumer wiring, e.g., extension cords, appliance cords and appliances. Generally, upon detection of an arc, it is desirable to open the circuit in which the arc is detected. Although arc detection is desirable, some known residential circuit breakers are large and expensive, which often precludes their use.
For example, some known residential circuit breakers that include integral arc detection units typically include a separate power supply, sometimes referred to in the art as a xe2x80x9cpig tailxe2x80x9d, to supply power to the arc detection electronics and a separate over-current trip unit. Such power supplies and trip units may be physically large. In order to include the power supply and trip unit within the circuit breaker enclosure, the circuit breaker housing typically must be increased in size from, for example, a 0.5xe2x80x3 form factor housing to a 1.0xe2x80x3 form factor housing.
The size of the breaker housing sometimes prevents such breaker from being used in at least some residential applications due to space constraints. Moreover, increased housing size also results in increased breaker cost.
It would be desirable to provide a power supply and trip unit for use in residential circuit breakers that perform the necessary functions and yet are relatively small in physical size. It also would be desirable to provide such power supply and trip unit in a form that is simple to fabricate and low in cost.
An instantaneous trip power transformer particularly well suited for residential circuit breaker applications includes a transformer, in an exemplary embodiment, having a high current main outer winding conductor. The main outer winding conductor may be wound to have one or more turns and provides the main breaker contact current path. The main outer winding conductor also serves as the primary winding for the power transformer to provide power to the breaker electronic components.
The transformer also includes a secondary winding configured to provide power to trip circuit electronic components. The secondary winding is wound to have a substantially cylindrical shape with a bore therethrough. The main outer winding conductor is wound around an outer surface of the secondary winding conductor. Leads are electrically coupled to, and extend from, the secondary winding conductor for supplying power to the trip circuit.
A third, or trip, winding is located within the secondary winding bore and is configured to trip the breaker under electronic control. Specifically, the third winding is wound to have a substantially cylindrical shape. Leads are electrically coupled to, and extend from, the third winding conductor to the trip circuit. A conducting cylinder is located in the third winding bore, and a tripping mechanism activation plunger is at least partially located in the cylinder and extends from one end of the third winding conductor. The plunger is mechanically coupled to a spring loaded switch that, in turn, spans the breaker main contact, as is well known in the art.
Prior to operation, the primary winding conductor is electrically coupled between a power supply, e.g., an AC power line, and the electronic components of the circuit breaker. The secondary conductor leads are electrically coupled to the trip circuit for supplying power thereto, and the third winding leads are electrically coupled to the electronic trip circuit.
In operation, current flows through the primary winding conductor and the primary winding conductor serves as the main breaker current path. Current induced in the secondary winding conductor from the primary winding conductor is utilized to power the trip circuit components. Under normal tripping conditions, the trip circuit activates the third coil with energy stored, for example, in a capacitor. The DC field from the third winding conductor is superimposed on the AC field generated by the primary winding conductor. As a result, the plunger activates the mechanical spring loaded switch.
In the event of a high current, e.g., a short circuit or its equivalent, in primary winding conductor 14, the increase in magnetic force of the primary winding conductor field activates tripping plunger 32 so that the plunger moves from the switch closed, i.e., plunger 32 inactivated position 37, to the switch open, i.e., plunger 32 activated position 38. The current level at which tripping plunger 32 moves from the inactivated to the activated position is selectable, and usually the high current is designated as a current in the range of 110 amps to 170 amps for a 15 amp or 20 amp circuit breaker. When plunger 32 is in the activated position, the breaker is xe2x80x9ctripped.xe2x80x9d Such tripping of the breaker is provided without requiring any control signals from the trip circuit. Rather, when a high current condition exists in primary winding conductor 14, plunger 32 is tripped due to the increase in force of the primary winding conductor AC field.
The above described integral trip coil and power transformer provides the important advantage of performing the required functionality, i.e., power supply and high current instantaneous trip, yet is small in size. Rather than using a 1xe2x80x3 form factor housing for a residential circuit breaker, a smaller size housing can be utilized. In addition, the transformer is not difficult to fabricate and is not expensive.